An Example Common Lisp Function Definition

Consider the binary trees x and y below.

In Lisp, x is written as the list '(A B) or, equivalently, as
'(A B . NIL). Similarly, y may be written '(C D E).
Suppose we wish to replace the right-most tip of x
by the entire tree y. This is denoted (app x y), where app
stands for ``append''.